1. Field of the Invention
The present invention relates to a filter for liquids, and more particularly to providing a highly efficient mechanical type self-cleaning filter.
2. Description of the Related Art
Conventionally, many types of self-cleaning mechanical filters are used to remove suspended solids from solid-entrained liquids. One type among those filters is a cylindrical screen filter. In a cylindrical screen filter, a filter element having a cylindrical shape and bearing numerous perforations is placed within a sealed housing having inlet and outlet connections. In this arrangement, liquid introduced into the housing flows from the inside of the filter element out through the perforations in the filter element. The filtered liquid then exits via the outlet of the housing. As a result, the solid material strained from the liquid flowing through the filter is retained on the inside of the filter element.
The filter is designed such that the retained solids are periodically discharged, using a back flushing liquid, through a manual or automatic exhaust valve opened to atmosphere. To achieve self-cleaning of the filter element, two different methods using suction nozzle mechanisms are typically employed to create suction power for drawing away the retained solids from the filter element for expulsion through the open exhaust valve. According to one of the methods, suction scanning of the inside surface of the filter element is carried out by a plurality of nozzles, typically having narrow slit shapes, which are arranged over the entire length of the filter element area. The nozzles simply rotate closely along the inside surface of the filter element when a self-cleaning cycle is started. According to the other method, a plurality of nozzles (usually of round shapes) scan the screen inside surface in a spiral movement during each self-cleaning cycle. This latter method employing a spiral motion may require a more complex structure but assures a stronger suction power compared to the former method, because the inlet area of the suction nozzles can be designed much smaller in the latter method than in the former method.
However, when using the above-described suction nozzle mechanisms, self-cleaning efficiency and effectiveness are seriously affected by the nature of particles being filtered. A self-cleaning mechanical filter configured using the above-described mechanism may fail to serve its function when treating liquids that contain a high load of organic materials such as algae, fiber, bacteria, and other soft natural particulates. For example, “white water” from paper mills contains large quantities of fiber contaminants and dissolved soft materials. When such liquids are filtered, materials wholly or partially embedded in the filter element may not be sufficiently removed by the self-cleaning process. Further, a phenomenon referred to as the “back clogging phenomenon” may occur, in which deposits form on the outer wall of the filter element to completely clog the filter element.